A communication system to be mounted on a car is conventionally adopted in which communication lines (hereinafter referred to as bus) connected to electronic control units (ECU: Electronic Control Unit) controlling the operation of appliances mounted on the car are connected with a relay connection unit such as a gateway to send and receive messages among the ECUs.
In recent years, with an increase in the function of the car, the number of the ECUs has increased and the number of messages to be sent and received among the ECUs has rapidly increased. Therefore there is a demand that messages are efficiently sent and received among the ECUs and that a communication load factor of the bus is decreased.
To decrease the communication load, it is conceivable that stopping the transmission of an incomplete message in which an error has occurred is effective. In this case, an ECU to which the message is to be sent is incapable of periodically receiving the message.
The error message is generated in a case in which an improbable sudden change has occurred in data relating to a car speed, temperature, illuminance, rainfall, and the like; a case in which data exceeds an ordinary set range; and a case in which an improbable combination is made among a plurality of data.
Thus in Japanese Patent Application Laid-Open No. 2002-16614 (patent document 1), there is proposed the gateway which does not transfer an incomplete message in which an error has occurred.
But in the gateway proposed in the patent document 1, whether the message is to be transferred is determined after judging whether an error has occurred by checking the content of the message. Therefore unless the message is entirely received, it is impossible to determine whether the message is to be transferred.
When it is determined that the message is not transferred, an ECU at a reception side is incapable of periodically receiving the message periodically sent and received among the ECUs.
Further the gateway is capable of dealing with a store & forward method of starting transmission after the message is entirely received, but is incapable of dealing with a cut-through method, having a favorable transfer efficiency, of determining a destination to which the message is transferred when the ID (identifier) is received before the message is entirely received and starting transmission. Thus the gateway is capable of decreasing the communication load factor of the bus, but has room for improvement in sending and receiving the message efficiently.    Patent document: Japanese Patent Application Laid-Open No. 2002-16614